1. Field of the Invention
This invention relates to a cap of a container for a photographic film cartridge.
2. Description of the Prior Art
A conventional cap of the container for a photographic film cartridge was usually made of high-pressure branched low-density polyethylene (LDPE) resin which is 0.927 g/cm.sup.3 density (ASTM D 1505) and 4.0 g/10 minutes in melt index (ASTM D1238, at 190.degree. C., MI). This resin is superior in flexibility at a low temperature, fitness of cap and body, compressive strength, and with small burr and gate marks.
Moreover, the inventor disclosed a cap of a container for a photographic film having 1200 to 4000 kg/cm.sup.2 of a bending rigidity (ASTM D-747) made of LDPE resin which is 0.918 to 0.930 g/cm.sup.3 in density and 7 to 40 g/10 minutes, in MI containing 0.05 to 1 wt. % of fatty amide lubricant in Japanese Patent KOKAI No. 62-291639. Another cap of a container for a photographic film disclosed in Japanese Patent KOKAI No. 63-113453 is made of LDPE resin having 0.920 to 0.935 g/cm.sup.3 in density, 10 to 50 g/10 minutes in MI and 3.8 to 8.5 in molecular weight distribution and containing 0.01 to 0.9 wt. % of a lubricant.
However, in the case of using the LDPE resin, molding troubles, such as a short shot of the molten resin, warp, twist and deformation, frequently occur. Its insufficient heat stability was also a problem. That is, the LDPE resin residing in a continuous molding machine at its screw, manifold, hot runner or other places was gradually colored brown or dark brown by heat. This colored resin was gradually extruded to cause coloring troubles.
Particularly in a transparent or translucent cap not colored. The occurrence of coloring trouble is at a high rate, i.e. 3 to 10%. As a result, total inspection of the molded caps was e necessary. Moreover, in the case of a colored cap, the yellowed resin in a state of lumps stayed at the nozzle portion, and the resin flow came to be inferior resulting in a short shot, and in a particular case causing the lack, of the molten resin.
In the caps disclosed in Japanese Patent KOKAI Nos. 62-291639 and 63-113453 by the inventor, the above problems were solved, but some problems described below occurred caused by using a LDPE resin and a lubricant. That is, the fitting strength decreases at high temperature, for example, 2.5 kg at -10.degree. C., 1.8 kg at 20.degree. C., 0.8 kg at 35.degree. C. and 0.5 kg at 50.degree. C., that is, the fitting strength at 50.degree. C. is 1/5 of that at -10.degree. C. As a result, the cap is liable to detach by the water vapor pressure in the container. Other problems also occurred such as deformation under heat, scratches and deformation generated during high speed mass molding or pneumatic transportation, scratches and abrasion by collisions and rubbing with other bodies, the increase of the blending cost of a lubricant, the adhesion of dust and white powder to a photographic film caused by bleeding out of the lubricant, gradual increase of haze and gradual color change. Moreover, in the case that the content of the lubricant is insufficient or the lubricant is not uniformly dispersed, the release of the cap from the mold or the deformation of the cap are liable to occur unless the molding cycle is extended.